A 4-Ton System Delivering 1,080 CFM With Two Hot Bedrooms
A 4-ton R-410A split system in a 2,000 ft² single-story ranch is rated for 1,600 CFM. The homeowner says the two far bedrooms stay 4–5°F warmer than the rest of the house. You measure return static at −0.55 in. w.c. and supply static at +0.45 in. w.c. at the air handler, with a 0.35 in. w.c. drop across a wet evaporator coil. The 20×25×1-inch MERV 11 filter reads 0.28 in. w.c. drop, and the return uses a single filter grille. Fan performance data for the current tap predicts about 1,080 CFM at that total static. Return air is 76°F dry bulb / 62°F wet bulb, supply air is 54°F, superheat is 12°F, subcooling is 9°F, and outdoor ambient is 92°F. Ductwork is a 14-inch flex trunk with long runs to the far bedrooms, and register airflow there is noticeably weak.
1. Is the complaint refrigerant-side or air-side? Justify with the numbers before recommending any repairs.
Read the worked answer
Air-side, and the refrigerant readings are the proof.
Refrigerant side:
- Superheat 12°F and subcooling 9°F at 92°F ambient are both inside normal ranges for a TXV system. There is no starving, flooding, or liquid-back signature.
- A 22°F split (76 − 54) looks impressive and is exactly the trap. High split with low airflow is a symptom of an airflow problem, not evidence of extra capacity.
Air side:
- Total external static = 0.55 + 0.45 = 1.0 in. w.c. That is high for a residential air handler, and the fan table predicts only 1,080 CFM.
- 1,080 / 1,600 = 68% of rated airflow, or 270 CFM/ton. Below about 350 CFM/ton the coil runs cold, the split widens, and the coil is at risk of icing.
- The 0.28 in. w.c. filter drop alone is a large fraction of the budget and points at the single 20×25 filter grille as a major restriction.
- Long 14-inch flex runs with weak register airflow explain why the far bedrooms lag even when the equipment is working.
Do not add or remove refrigerant, and do not replace the compressor. Fix static pressure and distribution first.
2. Estimate the sensible capacity actually being delivered and compare it with the nominal rating.
Read the worked answer
Using the sensible heat equation: Btu/h = CFM × 1.08 × ΔT.
1,080 × 1.08 × 22 = 25,661 Btu/h sensible, roughly 25,700 Btu/h.
A nominal 4-ton system is 48,000 Btu/h total, with a sensible fraction of about 0.70–0.75 at 76°F/62°F entering conditions, or roughly 34,000 Btu/h sensible. Delivered sensible capacity is therefore about 75% of nominal, and that assumes the 22°F split is real; it is inflated by low airflow, so the true loss is worse than the arithmetic suggests.
This is the key comparison for the report: the equipment is not broken, it is starved of air. Restoring 1,600 CFM at the same entering conditions would raise sensible capacity toward the nominal 34,000 Btu/h even if the split narrows to 19–20°F. A narrower split with more airflow is better performance, not worse.
3. Identify the specific sources of static pressure and the distribution defects, and describe the corrective sequence.
Read the worked answer
Static pressure sources, ranked by the measurements:
- Filter and return grille: 0.28 in. w.c. across a single 20×25 filter is high; face velocity is excessive. Return static of −0.55 in. w.c. confirms the return path is the bottleneck.
- Wet evaporator coil: 0.35 in. w.c. is normal-to-high; verify it is clean and the condensate is draining.
- Duct system: the remaining pressure is in supply duct, fittings, and long flex runs.
Distribution defects: a 14-inch flex trunk cannot carry 4 tons of air over long runs, and flex with sagging, tight bends, or excess length adds equivalent length and chokes the far-end registers.
Corrective sequence:
- Increase return capacity: add a second return grille or enlarge to a media cabinet with more face area; target filter drop under 0.15 in. w.c.
- Re-measure total static at the air handler and confirm airflow from the fan table; re-tap the blower only after the duct restrictions are reduced, so the motor is not overloaded.
- Replace long, kinked flex runs with smooth metal or short straight flex, increase trunk size, and add balancing dampers so the far bedrooms receive their share.
- Recheck split, superheat, and subcooling after airflow is restored; charge may need a final trim only if subcooling falls outside target at the corrected airflow.
- Document room-by-room temperatures before and after.